Semiconductor wafers, in particular of monocrystalline silicon, are needed as basic materials for the production of electronic components. The manufacturers of such components require that the semiconductor wafers have as far as possible planar and plane-parallel surfaces. In order to meet this requirement, the semiconductor wafers are subjected to a series of processing steps which improve the planarity and plane-parallelism of the sides and reduce their roughness. In the scope of this processing, one or more polishing steps are usually carried out.
Double-side polishing (DSP), in which both surfaces (front side and back side) of the semiconductor wafer are simultaneously polished in the presence of a polishing agent in the form of a suspension (also referred to as a slurry), is particularly suitable. During the double-side polishing, the semiconductor wafer together with further semiconductor wafers is placed in a gap between a lower polishing pad and an upper polishing pad. This gap is referred to as the working gap. Each of the polishing pads covers a corresponding lower or upper polishing plate. During the double-side polishing, the semiconductor wafers lie in recesses of carrier disks which guide and protect them. The carrier disks are externally toothed disks, which are arranged between an inner and an outer toothed wheel or pin gear of the polishing device. A toothed wheel or pin gear will be referred to below as a drive gear. During the polishing process, the carrier disks are set in rotation about their own axis and simultaneously in a revolving movement about the axis of the polishing device by rotation of the inner drive gear or by rotation of the inner and outer drive gears. Furthermore, the polishing plates are usually also rotated about their axes. For the double-side polishing, this results in characteristic so-called planetary kinematics, in which a point on a side of the semiconductor wafer describes a cycloid path on the corresponding polishing pad.
One main purpose of the double-side polishing of semiconductor wafers is to improve the global and local geometry. In this case, a semiconductor wafer which is as planar as possible is intended to be produced without edge roll-off in an economical process. This can be achieved by interaction of the various process parameters in the polishing process. One important parameter is the polishing gap between the upper and lower polishing pads. In this context, the conditioning of the polishing pad surfaces plays a crucial role for the polishing process. During the conditioning, on the one hand the surface of the polishing pad is cleaned (dressing) and on the other hand slight material abrasion is induced in order to impart the desired—generally as planar as possible—geometry to the polishing pad surface (truing).
Usually, the polishing pads are in this case processed with conditioning disks whose surfaces facing toward the polishing pad are coated with abrasive particles, for example diamond. The conditioning disks have external teeth, so that they can be placed like a carrier disk on the lower polishing pad, the external teeth engaging with the inner and outer drive gears. The upper polishing plate is placed on the conditioning disks, so that the conditioning disks lie in the working gap between the upper and lower polishing pads. During the conditioning, similar kinematics are used as in the polishing. The conditioning disks therefore move during the conditioning process with planetary kinematics in the working gap and process the upper or lower polishing pad, or both polishing pads, depending on whether conditioning disks coated with abrasive on one or both sides are used.
With this standard method, a plane-parallel working gap can be achieved. Furthermore, unevennesses on the polishing pad surfaces can be removed. It has been assumed that an optimal geometry of the polished semiconductor wafers can be achieved by a working gap which is as plane-parallel as possible.
US2012/0028547A1 describes a possibility of imparting a correspondingly concave or convex surface shape to the polishing pads by using conditioning tools with a convex or concave surface. The conditioning tools, like the semiconductor wafers to be polished, are placed in the recesses of the carrier disks. In this way, the geometry for the polishing pad surface can be adjusted in such a way that the geometry of the polished semiconductor wafers is improved. For example, it is indicated that a pronounced biconcave configuration of the polished semiconductor wafers can be avoided by concave polishing pad surfaces (i.e. a small width of the polishing gap at the inner and outer edges of the polishing plates and a larger gap width at the radial center of the polishing plates).
However, it has been found that even this measure is not sufficient in order to satisfy the increasing requirements for the geometry of the polished semiconductor wafers.